Electrical distribution and current collecting device for high speed vehicles

ABSTRACT

A high speed vehicle is electrically feeded by a movable collector head running in a passageway defined by a plurality of regularly spaced apart multi-phase energized conductive rails. To improve the guiding characteristics of the collector head, mechanical coupling means are provided between the driven collector head and the driving vehicle such that the resultant of the force exerted by the coupling means on the collector head is substantially directed in the direction of movement of the collector head.

Apr. 24, 1973 United States Fatent Payen S T N m n m w mm C! 9 R m N U M 1 1 R m WE Am V WE ID W BNE IIL RTC Tm a m DLV LO AC m E RNP T C H E LU ECH 4 H 8 bl 3,571,532 3/1971 Falque ...19l/45A 3 Greno 3,405,240 10/1968 Knbur ...,...........................191 45A 3,610,844 10/1971 Blanchard..............................191/45 [75] Inventor: Jean-Pol Payen,

France [73] Assignee: Merlin Gerin, Societe Anonyme, Grenoble, France Oct. 6, 1970 Appl. No.: 78,551

Primary ExaminerGerald M. Forlenza Assistant Examiner-Richard A. Bertsch Attorney-Stevens, Davis, Miller & Mosher 22 Filed:

[57] ABSTRACT A high speed vehicle is electrically feeded by a movable collector head running in a passageway defined by a plurality of regularly spaced apart multi-phase ener- Foreign Application Priority Data gized conductive rails. To improve the guiding characteristics of the collector head, mechanical coupling means are provided between the driven collector head and the driving vehicle such that the resultant of the force exerted by the coupling means on the collector 456 277 11 I33 433 900 can CC.C nnn 3H3 rrrl FFF mm $99 11 1l w DSS [52] US. ...191/49, 191/45 R, 191/59 [51] Int. Cl. 5/08 head is substantially directed in the direction of movement of the collector head.

5 9, .0 5 m5 4 "3 m3 m9 2 @2 mp 1 9 1 0 d 1 e i F l 00 5 8 Claims, 10 Drawing Figures Patented April 24, 1973 4 3,729,599

4 Sheets-Sheet l M9. z ii o I I 2 41 z I Ao Ma 4 4'20 I i M6 422. m m m 46.0 4a m Patented April 24, 1973 4 Sheets-Sheet 2 Fig.3

Patented April 24, 1973 4 Sheets-Sheet 5 Patented April 24, 19,73

4 Sheets-Sheet 4.

ELECTRICAL DISTRIBUTION AND CURRENT COLLECTING DEVICE FOR HIGH SPEED VEHICLES This invention relates to an electric feed device for a high speed vehicle, such as a vehicle propelled by an electric motor (for instance a linear motor), whereby the vehicle is fed by a collector head running in a passageway defined by a plurality of multi-phase energized rail-shaped conductors extending along the path of the vehicle.

It is an object of the present invention to assure permanent electric contact at all speeds of displacement between the contact shoes of the collector head and the associated conductors so as to avoid any sparking and thus any wear or damage to the contact shoes or feed conductors.

Another object of the present invention is to provide a current collector head driving system which permits substantial oscillations of the vehicle, in particular oscillations in height resulting from variations in thickness of the fluid hovering cushion as a function of the load of the vehicle in the case of air cushion sustained vehicles.

Another object of the invention is to improve the collecting of polyphase currents from rigid conductors in the form of rails which extend along the path of the vehicle.

Still another object of the invention is to permit the guiding of a collector head, pulled by the vehicle, by the feed conductors in such a manner as to avoid any accidental or undesired derailment of the collector head the repercussions of which at high speed might be disastrous.

Another object of the invention is to permit the pulling of the collector head by the vehicle through a coupling system which does not produce at cruising speeds forces which disturb the balance of the collector head.

Another object of the invention is to provide a current collecting device which can function in either direction of displacement so as to avoid any reversing or modification at the end of the run.

Another object of the'invention is to provide a current collecting device and driving mechanism of outstanding simplicity and reliability of operation at any speed of travel of the vehicle.

Still another object of the invention is to improve the aerodynamic behavior of the collector head at high speeds.

Another object of the invention is to diminish the stresses in the collector head during its high speed movement in the passageway defined by the multi phase conductor.

These and other objects and advantages will become apparent upon reading of the following description of preferred embodiments of the present invention, which is given solely by way of illustration and not of limitation, and is shown in the accompanying drawings, in which:

FIG. 1 is a schematic, partial plan view of a current collecting system according to a first embodiment of the invention;

FIG. 2 is a cross-section along the line IIII of FIG. 1;

FIG. 3 is a view on a larger scale, partially in crosssection, of the collecting device of FIG. 1;

FIG. 4 shows schematically in elevation, on a larger scale, the mechanical connecting system between the vehicle and the collector head;

'FIG. 5 is a schematic end view ofa mechanical connection system between a vehicle and a collector head according to a second embodiment of the invention;

FIG. 6 is a partial elevational view, partially in crosssection of the system of FIG. 5;

FIG. 7 is a schematic view of the system of FIG. 6;

FIGS. 8 to 10 show other embodiments in views corresponding to the view of FIG. 7.

Referring to FIGS. 1 and 2, a vehicle which is supported by a cushion of compressed air which is developed between the bottom 112 of the vehicle and the base ofa track 114 having the cross-sectional shape of an inverted T is propelled along the track 114 by a linear electric motor (not shown), the armature part of which consists of the protruding plate 116 of the inverted T of the track 114 which is straddled by the vehicle 110. I

Such vehicles, which are well known to those skilled in the art, are particularly suitable for displacement at high speeds which may reach several hundred miles per hour, but it will be understood that the invention is in no way limited to a transport system of this type and that it also applies to vehicles having wheels.

The electric motor which is present on the vehicle 110 is fed with multi-phase electrical current by a collector head 14 which 7 slides within a track or passageway 10 defined by rail-shaped feed conductors 12, 12' and 12" which extend along the track 114 of the vehicle from which they are laterally spaced.

The multi-phase energized rails 12, 12 and 12" are stationary and supported by posts or supports (not shown) and each of them corresponds to one phase of the feed current of the propulsion motor of the vehicle. Within the track 10, which has the shape of a passageway defined by the triangle arrangement of the rails 12, 12' and 12", the collector head 14 is guided in its displacement by current collecting members 16, 16' and 16" carried by the collector head and arranged opposite the rails 12, 12 and 12" against which they slide in order to collect the electric current.

The guiding of the collector head 14 is assured exclu sively by the rails 12, 12' and 12" via collecting members 16, 16 and 16", the contact shoes of which assure simultaneously electric contact and mechanical contact between the collector head 14 and the track 10 formed of the rails 12, 12' and 12". It will be clear from the following description that the triangular arrangement of the rails 12, 12 and 12" makes it possible to assure permanent centering of the collector head towards the center of this triangle. Although the arrangement along an equilateral triangle shown in FIG. 3 seems most suitable, it is obvious that other arrangements could be utilized. In the case of four feed conductors, for instance, of a three-phase source comprising a neutral conductor, an arrangement of the rails in accordance with a quadrilateral, for instance a square or a parallelogram, could also be employed.

The current collecting members 16, 16' and 16" as well as the rails 12, 12' and 12" being all of the same construction, only one of them will be described below with reference to FIG. 3.

The rail 12, made of a conductive material such as aluminum, has a contact surface 18 of a concave, V-

shaped cross-section, made of stainless steel, in which fits the matching contact surface of a contact shoe or brush 22 under the action of a contact spring 24 which urges the shoe 22 towards the rail 12 and is inserted between the base of the shoe 22 and the body or web portion 14 of the collector head. The shoe 22 is slidably mounted in a trough-shaped guide 26, the lips of which are spaced from the rail 12 so as to provide a space which prevents any accidental contact between these parts during the displacement of the collector head 14. The guide 26 is part of or rigidly connected with an elongated U-shaped member 311 rigidly fastened to the body 14 by partial embedment and by fastening screws (not shown) or in any other manner. The profiled strip 30 is made of conductive material and advantageously supports a plurality of contact shoes 22 which are longitudinally spaced apart so as to increase the contact surfaces and retain correct collecting of the current even in the case of bouncing of one of the shoes. This staggering also favors the guiding of the collector head 14 over its entire length by contact alone of the contact shoes. At each phase there corresponds one shaped supporting member 30, the collector head's body being made of insulating material. The electrical connection between the shoe 22 and the channel member 30 is assured in the customary manner by a braided cable 32.

The guide 26 is housed entirely within the recess defined laterally by the legs 34, 36 of the U-shaped member 30 the ends of which are substantially flush with the plane of contact of the shoe 22 and of the rail 12. A screen or covering 38 which may advantageously be of insulating material lines the inside of the U- shaped member 30 and its front edges 39, 40 extend with a flare so as to surround the rail 12 with clearance over a limited height.-

The rail 12 is also lined by a sheath 42 the lips 44, 46 of which protrude in the direction of the collector head 14 and fit with play in the interstices provided between the side walls ofthe covering 38 and the guide 26. The rail 12 has a thinned section 48 which permits attachment of the sheath 42 by clipping. The sheath 42 is of insulating material, advantageously semi-rigid, such as a plastic or synthetic material, and it will be noted that the rail 12 is almost entirely surrounded by this insulator, which avoids any accidental contact with the conductors which are under voltage. The spacing provided between the lips 44 or 46, the guide, 26 on the one hand and the covering 38 on the other hand are sufficient to avoid any contact in normal operation.

The only mechanical or electrical contact between the collector head 14 and the rails 12, 12' and 12" is provided by the shoes 22 which are urged against the rails by the springs 24. The reaction of these springs 24 exerted on the collector head assures the centering of the collector head within the track 10 defined by the rails 12, 12' and 12". Any rotation of the collector head 14 around its own axis is counteracted by the V- shaped contact surface, for instance shown in FIG. 3, of the shoes 22 with the associated rails 12, 12 and 12". It is understood that this contact surface might be of any other configuration. In normal operation the guiding and positioning forces of the collector head 14 obtained from the shoes 22 are sufficient to maintain the collector head on its trajectory defined by the track 10.

A safety locking of the collector head 14 within the track 10 is assured by the lips 44, 46 and/or the edges 39, 40 of the covering 38 which take over the guiding of the collector head in case of a substantial deviation of'the collector head 14, for instance due to a substantial deformation of the track. One will thus avoid any derailment and liberation of the collector head 14 which might have catastrophic consequences at the high speeds attained. It is understood that this safety interlock could be produced in any other manner, for instance by a special shape of the rail 12 itself and of the supports of the shoe 22.

The electrical connections between the supports 30 of the collector head 14 and the vehicle are assured by connections which are incapable of transmitting any appreciable mechanical force, for instance by flexible connections, such as braids or suitable articulated systems. The collector head 14 is pulled by the vehicle 110 via a mechanical connection which is capable of compensating for the differences in position between the collector head and the vehicle resulting from a variation in thickness of the fluid supporting cushion. In accordance with the invention, the traction system is so arranged as not to apply to the collector head any external forces which might disturb the equilibrium of the collector head at cruising speed and in any event to limit these disturbing forces.

To the vehicle 110 there are fastened two coupling parts in the form of bent legs 118 and 120 the ends 122 and 124 extend between the conductors 12" and 12 and penetrate up to the central portion of the space defined by the conductors 12, 12' and 12". The couplings 118 and 120 are spaced in such a manner as to surround the collector head 14 at the front and the rear in the direction of travel, and they are arranged in the central portion of the vehicle 110 for reasons of symmetry but a different arrangement could be retained.

Referring more particularly to FIG. 4 which illustrates the manner of connection between the couplings and the collector head 14, it is seen that an orifice 126, 128 is arranged respectively in each leg 118, 120 near the ends 122, 124 on an axis parallel to the conductors 12, 12' and 12". At the center of each front surface 130, 132 of the collector head 14 there is articulatedly mounted a connecting rod 134 and 136 respectively which are extended by a rod 138 and 140 respectively passing through the orifice 126, 128. A swivel joint 142, 144 is interposed between the connectingrods 134, 136 and the rods 138, 140, the latter terminating in a stop 146, 148. A compression spring 150, 152 is placed over the rods 138, 140 and inserted between the stop 146, 148 and the leg 118, 120 so as to urge the rods 138, 140 into extended position.

It will be understood that the system of fastening by articulated connecting rods 134 and 136 and by sliding rods 138 and 140 permits any displacement of the collector head 14 with respect to the couplings 118, 120, which displacement, of course, is limited but sufficient to compensate for differences which may occur between the vehicle 110 and the collector head 14 in operation. The springs 150 and 152 urge'the rods 138, 140, the connecting rods 134, 136 and the collector head 14 in alignment in axial direction parallel to the axis of the vehicle and of the trajectory. The entire system is arranged in such a manner that this position, illustrated in FIG. 4, corresponds to the normal position of the vehicle 110 at cruising speed, oscillations of limited amplitude occurring around this central position. 7

The operation of the device in accordance with the invention is obvious:

The rails 12, 12 and 12" are connected to a source of polyphase current, for instance three-phase. and, via shoes 22 and connecting braids, they feed the motor of the vehicle 110 which is thus propelled along the track 114. In the case of displacement of the vehicle 110 in the direction indicated by the arrow in FIG. 1, the collector head 14 is pulled by the coupling 118 acting via the spring 150 on the rod 138 and the connecting rod 134. The pulling force exerted by the connecting rod 134 on the collector head 114 acts at the center of the latter in the direction of displacement when the vehicle 110 is positioned with respect to the collector head 14 in the manner shown in FIG. 4 and it is easy to understand that this traction force in no way affects the equilibrium of the collector head, which is guided and positioned by the shoes 22 in contact with the rails 12, 12' and 12". Upon a reversal in direction of travel, the role of the couplings 118 and 120 is, of course, changed, one of the couplings being furthermore possibly eliminated in the case of a vehicle having a single direction of travel. In the case of accidental disturbances or of travel at slow speed of the vehicle 110, or even when the latter is stopped, the position of the vehicle 110 with respect to the track 114 and to the track 10 of the collector head 14 may be different. These variations are compensated for by the coupling system consisting of articulated and slidable rods which connect the collector head 14 to the vehicle. The oblique position assumed by the tie rod 134 or 136 imposes a lateral component on the collector head 14 tending to bring the latter out of balance. This imbalance which lasts for a limited period of time or appears at low speeds of travel, is generally of a permissible value and capable of being taken care of by the customary guide system. By way of exception, the safety device can prevent the derailment of the collector head 14. I

In FIGS. 5 217 6, a connecting rod 216 protudes laterally from the vehicle 110 in such a manner that its free end is located above the feed track or passageway 10 defined by conductors 12, 12' and 12". At this free end is pivotally mounted at 216 the end of a vertical rod 218. On a horizontal pin 220 of the rod 218 there is articulated a pair of parallel levers 222, 222', and on a pin 224 which is at a different height there is pivoted a second pair of levers 226, 226. The levers 222, 222' and 226, 226 extend along and are capable of swinging in vertical planes close to the plane of vertical symmetry of the passageway 10 in normal operation. The opposite ends of the levers 222,222 and 226,226 are pivoted at points 228 and 230 respectively ofa support rod 232, the assembly being arranged so as to form an articulated parallelogram, designated generally as 231, which holds the rod 232 in vertical position parallel to the rod 218 whatever the deformations imposed on the articulated frame. The rod 232 terminates in a spherical ball 234 which fits in a matching socket 236 provided in the body of the collector head 14 so as to permit limited orientation in any direction of the collector head 14 around the ball 234. In the example shown in FIGS. 6 and 7, the housing for the ball 234 is arranged at the center of the collector head 14.

In operation, the connecting bar 216 moves with the vehicle along a path substantially parallel to the feed track 10, and this movement is transmitted by the vertical rod 218, the levers 222, 222' and 226, 226', the rod 232 and the ball joint 234 to the collector head 14 which slides along the track 10. The ball joint 234 permits free orientation of the collector head 14 with respect to the traction rod 232, such as the orientation imposed, for instance, by irregularities in the profile of the track 10. A vertical movement of the vehicle 110 resulting from a variation in the thickness of the supporting cushion causes a deformation of the articulated parallelogram 218, 222, 226, 232 so as to maintain the ball joint 234 at a constant height. The traction force is in all cases applied to the center of the collector head 14 which corresponds to the drag center, that is the center of application of the resultant of the motion opposing forces (aerodynamic forces; frictional forces due to contact friction of shoes), due to the symmetrical arrangement of the collector system, and no couple which disturbs the equilibrium of the collector head 14 is produced. The variation in inclination of the arms 222, 226 of the parallelogram modifies the vertical component applied to the collector head but within acceptable limits if their length has been selected accordingly. The weight of the collector head 14 furthermore compensates in part for this vertical component.

A lateral swinging of the vehicle 110 places the rod 218 out of alignment with the track 10, which lack of alignment is automatically compensated for by a slightly oblique position of the arms 222 and 226 as permitted by the articulation 217 of the rod 218 on the connecting rod 216 and by the ball joint 234. The lateral componentresulting from the oblique traction forces is practically negligible.

FIGS. 8, 9 and 10, which are similar to FIG. 7, illustrate schematically different embodiments of the invention which permit displacement in two directions of travel.

In FIG. 8 two articulated parallelogram or pantograph traction systems 231 and 231 are arranged symmetrically with respect to the central plane of the collector head 14. The sockets of the balls 234 and 234, provided in the vicinity of the ends of the collector head 14, are arranged in slides 236, 236 so as to permit the balls 234 and 234 to move towards and away from each other upon a deformation of one or both of the parallelograms 231, 231. The dead stroke or lost motion, due to the slides 236, 236', prevents a joint pulling action of the two pantographs 231 and 231, only the pantograph 231 being active upon displacement in the direction indicated by the arrow in the drawing, and vice versa.

FIG. 9 is similar to FIG. 8, except that the difference between the pantographs 231 and 231 is less, which in no way changes the operation of the current collecting.

FIG. 10 illustrates one manner of fastening to a single rod 218 two articulated parallelograms 238 and 238' which extend in opposite directions on opposite sides of the rod 218.

At high speed the aerodynamic forces acting on the elements of the traction pantograph can be substantial, particularly the forces of ascent or descent acting on the inclined arms 222, 226 of the parallelogram. By way of example, it will be noted that the pantograph 231 of FIG. 7 experiences an upward positive aerodynamic thrust upon displacement in the direction indicated by the arrow of FIG. 7, while the action on the pantograph 231 is in downward direct-ion. The collector head 14 is thus subjected to a pivoting torque which is a function of the speed of propulsion. In accordance with the invention, an inclined fixed wingprofile plate 240 is added to the pantograph 231, for instance on the lever 222, so as to produce a downward aerodynamic thrust of a value which substantially compensates for the aerodynamic thrust due to the levers 222 and 226. In identical manner a wing profile 240' is arranged with the pantograph 231 but with reverse inclination so as to produce an upward thrust. It will be understood that the disturbing effects are thus eliminated for any speed, whatever the direction of travel, the effects automatically reversing themselves with the direction of travel.

Such balancing profiles can, of course, be fastened at other suitable points of the system and be attached to any of the devices described above. The forces of gravity can also be compensated by a spring system which is well known per se and has not been shown in the drawing.

What is claimed is:

1. A multi-phase electrical distribution and current collecting system for a high speed vehicle movable along a track, comprising a movable collector head, a plurality of stationary multi-phase energizable rails of conductive material extending parallelly along said track and which are circumferentially regularly spaced apart to define a passageway for said collector head, said collector head comprising movable contact shoe means shaped for engagement with said rails in electrical contact relation, said collector head including bias means to resiliently urge said contact shoe means against said rails thereby centering and guiding said collector head along said passageway, and linkage means connecting said collector head to said vehicle, said linkage means being connected to said collector head in such a manner that the resultant driving force exerted by said linkage means on said collector head passes substantially through the drag center thereof and is substantially directed along the direction of movement of said collector head without applying any torque to the collector head tending to deviate said collector head from the equilibrium position thereof in said passageway.

2. A system according to claim 1, said linkage means comprising a pair of coplanar oppositely disposed pantographs, each pantograph having first pivot connections to said vehicle and second pivot connections to said collector head, said second pivot connections of said pantographs being longitudinally spaced apart on said collector head.

3. A system according to claim 2, said second pivot connections comprising longitudinal lost motion means.

4. A system according to claim 2, said pantographs being disposed back to back having a common side pivotally connected to said vehicle. t

S. A system according to claim 1, further comprlslng means shaped for engagement with said rails in electrical contact relation, said rails guiding said collector head along said passageway, and linkage means comprising a pair of legs laterally depending from said vehicle connecting said collector head to said vehicle and tie means extending longitudinally of said rails to tighten said collector head longitudinally between the free endsof said legs, said linkage means being adapted to apply to said collector head a resultant driving force substantialiy directed along the direction of movement ofsaid collector head in said passageway.

7. A system according to claim 6, said tie means comprising a plurality of swivel connected tie rods, and spring means to tighten said tie rods and to resiliently connect said tie rods to said free ends of said legs.

8. A multi-phase electrical distribution and current collecting system for a high speed vehicle movable along a track, comprising a movable collector head, a plurality of stationary multi-phase energizable rails of conductive material extending parallelly along said track and defining a passageway for said collector head, said rails being substantially equidistantly spaced apart to define an equilateral prism, said collector head comprising contact shoe means shaped for engagement with said rails in electrical contact relation, said rails guiding said collector head along said passageway, and linkage means connecting said collector head to said vehicle, said linkage means being adapted to apply to said collector head a resultant driving force substantially directed along the direction of movement of said collector head in said passageway, said collector head comprising a web portion, a longitudinally extending peripheral channel member for each of said rails secured to said web portion and facing the corresponding rail, a pair of lateral screen members of insulating material secured to each channel member and projecting outwardly from said channel member a predetermined length, a cover member of insulating material covering each rail and having a pair of protruding lips, said lips and said screen members overlapping each other a substantial length, a plurality of distinct contact shoes movable in each channel and adapted to engage mating surfaces of said rails, substantial radially extending guiding means for each contact shoe, and spring means to urge said contact shoes radially into engagement with said rails. 

1. A multi-phase electrical distribution and current collecting system for a high speed vehicle movable along a track, comprising a movable collector head, a plurality of stationary multi-phase energizable rails of conductive material extending parallelly along said track and which are circumferentially regularly spaced apart to define a passageway for said collector head, said collector head comprising movable contact shoe means shaped for engagement with said rails in electrical contact relation, said collector head including bias means to resiliently urge said contact shoe means against said rails thereby centering and guiding said collector head along said passageway, and linkage means connecting said collector head to said vehicle, said linkage means being connected to said collector head in such a manner that the resultant driving force exerted by said linkage means on said collector head passes substantially through the drag center thereof and is substantially directed along the direction of movement of said collector head without applying any torque to the collector head tending to deviate said collector head from the equilibrium position thereof in said passageway.
 2. A system according to claim 1, said linkage means comprising a pair of coplanar oppositely disposed pantographs, each pantograph having first pivot connections to said vehicle and second pivot connections to said collector head, said second pivot connections of said pantographs being longitudinally spaced apart on said collector head.
 3. A system according to claim 2, said second pivot connections comprising longitudinal lost motion means.
 4. A system according to claim 2, said pantographs being disposed back to back having a common side pivotally connected to said vehicle.
 5. A system according to claim 1, further comprising ball and socket means connecting said linkage means to said collector head substantially at the drag center thereof.
 6. A multi-phase electrical distribution and current collecting system for a high speed vehicle movable along a track, comprising a movable collector head, a plurality of stationary multi-phase energizable rails of conductive material extending parallelly along said track and defining a passageway for said collector head, said collector head comprising contact shoe means shaped for engagement with said rails in electrical contact relation, said rails guiding said collector head along said passageway, and linkage means comprising a pair of legs laterally depending from said vehicle connecting said collector head to said vehicle and tie means extending longitudinally of said rails to tighten said collector head longitudinally between the free ends of said legs, said linkage means being adapted to apply to said collector head a resultant driving force substantially directed along the direction of movement of said collector head in said passageway.
 7. A system according to claim 6, said tie means comprising a plurality of swivel connected tie rods, and spring means to tighten said tie rods and to resiliently connect said tie rods to said free ends of said legs.
 8. A multi-phase electrical distribution and current collecting system for a high speed vehicle movable along a track, comprising a movable collector head, a plurality of stationary multi-phase energizable rails of conductive material extending parallelly along said track and defining a passageway for said collector head, said rails being substantially equidistantly spaced apart to define an equilateral prism, said collector head comprising contact shoe means shaped for engagement with said rails in electrical contact relation, said rails guiding said collector head along said passageway, and linkage means connecting said collector head to said vehicle, said linkage means being adapted to apply to said collector head a resultant driving force substantially directed along the direction of movement of said collector head in said passageway, said collector head comprising a web portion, a longitudinally extending peripheral channel member for each of said rails secured to said web portion and facing the corresponding rail, a pair of lateral screen members of insulating material secured to each channel member and projecting outwardly from said channel member a predetermined length, a cover member of insulating material covering each rail and having a pair of protruding lips, said lips and said screen members overlapping each other a substantial length, a plurality of distinct contact shoes movable in each channel and adapted to engage mating surfaces of said rails, substantial radially extending guiding means for each contact shoe, and spring means to urge said contact shoes radially into engagement with said rails. 